DESCRIPTION

ieee80211_amrr is an implementation of the AMRR transmit rate control
algorithm for drivers that use the net80211 software layer. A rate
control algorithm is responsible for choosing the transmit rate for each
frame. To maximize throughput algorithms try to use the highest rate
that is appropriate for the operating conditions. The rate will vary as
conditions change; the distance between two stations may change,
transient noise may be present that affects signal quality, etc.
ieee80211_amrr uses very simple information from a driver to do it’s job:
whether a frame was successfully delivered and how many transmit attempts
were made. While this enables its use with virtually any wireless device
it limits it’s effectiveness--do not expect it to function well in
difficult environments and/or respond quickly to changing conditions.
ieee80211_amrr requires per-vap state and per-node state for each station
it is to select rates for. The API’s are designed for drivers to pre-
allocate state in the driver-private extension areas of each vap and
node. For example the ral(4) driver defines a vap as:
struct rt2560_vap {
struct ieee80211vap ral_vap;
struct ieee80211_beacon_offsets ral_bo;
struct ieee80211_amrr amrr;
int (*ral_newstate)(struct ieee80211vap *,
enum ieee80211_state, int);
};
The amrr structure member holds the per-vap state for ieee80211_amrr and
ral(4) initializes it in the vap create method with:
ieee80211_amrr_init(&rvp->amrr, vap,
IEEE80211_AMRR_MIN_SUCCESS_THRESHOLD,
IEEE80211_AMRR_MAX_SUCCESS_THRESHOLD,
500 /* ms */);
The node is defined as:
struct rt2560_node {
struct ieee80211_node ni;
struct ieee80211_amrr_node amrr;
};
with initialization done in the driver’s iv_newassoc method:
static void
rt2560_newassoc(struct ieee80211_node *ni, int isnew)
{
struct ieee80211vap *vap = ni->ni_vap;
ieee80211_amrr_node_init(&RT2560_VAP(vap)->amrr,
&RT2560_NODE(ni)->amrr, ni);
}
Once ieee80211_amrr state is setup, transmit rates are requested by
calling ieee80211_amrr_choose() in the transmit path; e.g.:
tp = &vap->iv_txparms[ieee80211_chan2mode(ni->ni_chan)];
if (IEEE80211_IS_MULTICAST(wh->i_addr1)) {
rate = tp->mcastrate;
} else if (m0->m_flags & M_EAPOL) {
rate = tp->mgmtrate;
} else if (tp->ucastrate != IEEE80211_FIXED_RATE_NONE) {
rate = tp->ucastrate;
} else {
(void) ieee80211_amrr_choose(ni, &RT2560_NODE(ni)->amrr);
rate = ni->ni_txrate;
}
Note a rate is chosen only for unicast data frames when a fixed transmit
rate is not configured; the other cases are handled with the net80211
transmit parameters. Note also that ieee80211_amrr_choose() writes the
chosen rate in ni_txrate; this eliminates copying the value as it is
exported to user applications so they can display the current transmit
rate in status.
The remaining work a driver must do is feed status back to ieee80211_amrr
when a frame transmit completes using ieee80211_amrr_tx_complete().
Drivers that poll a device to retrieve statistics can use
ieee80211_amrr_tx_update() (instead or in addition).